Invasive crabs flourish in Marin lagoon, despite eradication efforts

Kate Bimrose measures a European green crab trapped and removed from Seadrift Lagoon in Stinson Beach.

Kate Bimrose measures a European green crab trapped and removed from Seadrift Lagoon in Stinson Beach.

Photo: Paul Chinn, The Chronicle

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Ian Pritchard, a marine biologist with the UC Davis Bodega Marine Laboratory, returns to shore with non-native European green crabs collected in underwater traps from Seadrift Lagoon in Stinson Beach, Calif. on Tuesday, Aug. 15, 2017. The researchers logged 333 of the species trapped on Tuesday alone. The invasive crustaceans have been plaguing the Pacific coast from Monterey Bay north to British Columbia for the past several years. less

Ian Pritchard, a marine biologist with the UC Davis Bodega Marine Laboratory, returns to shore with non-native European green crabs collected in underwater traps from Seadrift Lagoon in Stinson Beach, Calif. on ... more

Photo: Paul Chinn, The Chronicle

Image 3 of 4

Kate Bimrose measures a non-native European green crab trapped and removed from Seadrift Lagoon in Stinson Beach, Calif. on Tuesday, Aug. 15, 2017. Marine biologists recorded 333 of the species trapped on Tuesday alone. The invasive crustaceans have been plaguing the Pacific coast from Monterey Bay north to British Columbia for the past several years. less

Kate Bimrose measures a non-native European green crab trapped and removed from Seadrift Lagoon in Stinson Beach, Calif. on Tuesday, Aug. 15, 2017. Marine biologists recorded 333 of the species trapped on ... more

Photo: Paul Chinn, The Chronicle

Image 4 of 4

European green crabs trapped and removed from Seadrift Lagoon in Stinson Beach, Calif. by marine biologists are kept in a bin before they're measured and logged on Tuesday, Aug. 15, 2017. The researchers logged 333 of the species trapped on Tuesday alone. The invasive crustaceans have been plaguing the Pacific coast from Monterey Bay north to British Columbia for the past several years. less

European green crabs trapped and removed from Seadrift Lagoon in Stinson Beach, Calif. by marine biologists are kept in a bin before they're measured and logged on Tuesday, Aug. 15, 2017. The researchers logged ... more

Photo: Paul Chinn, The Chronicle

Invasive crabs flourish in Marin lagoon, despite eradication efforts

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The bucket of crabs on the shoreline of Seadrift Lagoon in Marin County was a bonanza for 7-year-old Luca McLaughlin, who giddily handled as many as he could, but the pile of decapods represent a puzzling menace to marine biologists.

European green crabs have flourished in the man-made lagoon in a wealthy gated community in northern Stinson Beach, and scientists can’t seem to get rid of the pinching invaders.

“It’s actually one of the favorite pastimes of the kids, catching crabs,” said Daniel McLaughlin, 41, who watched as his son and a friend chased crustaceans in the shallows this week in an attempt to add to the haul.

Unfortunately, researchers say, the lagoon’s young visitors will never be able to catch enough of the greenish shellfish, considered one of the world’s worst invasive species. The crabs, which cause an estimated $22 million in damage to U.S. fisheries and marine habitat every year, are more dense in Seadrift Lagoon than any other place on the West Coast.

Efforts to eradicate them here have, to everyone’s astonishment, actually increased their population.

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“They have substantial impacts on local fauna, reducing native clams and native shore crabs by well over 90 percent,” said Ted Grosholz, a professor of environmental science and policy at UC Davis. “If we saw the densities we have seen in Seadrift in the rest of the Bay Area, they would have a significant impact on the shellfish industry.”

Grosholz and his UC Davis colleagues have been working since 2009 with the Smithsonian Environmental Research Center in Marin County, the Greater Farallones National Marine Sanctuary and Portland State University on ways to rid the lagoon of green crabs.

The project, which received funding from the Pacific States Marine Fisheries Station in Portland, became critical as the number of green crabs in the mile-long lagoon grew to well over 100,000 since the species was discovered in San Francisco Bay in 1989.

Connected by culverts to Bolinas Lagoon — the presumed source of the crabs — Seadrift Lagoon is an isolated body of water with no predators to keep the population down. The idea behind the project was to figure out the best way of getting rid of the creatures, then copy the method in other areas.

But things did not go as planned. Grosholz said several years of intensive trapping shrunk the crab population to about 8,000. Then in 2013, the number exploded to about 300,000 — a turn of events that Grosholz likened to “The Sorcerer’s Apprentice,” a 1797 poem relating to magic and unanticipated consequences, popularized in a segment of the animated 1940 Disney film “Fantasia.”

“It was discouraging,” said Andrew Chang, a program leader for the Smithsonian Environmental Research Center. “We put all this effort into it and, hmmm, look what happened.”

Photo: Paul Chinn, The Chronicle

Kate Bimrose (left) and Monika Krach examine a European green crab. They have flourished in Seadrift Lagoon.

The researchers determined that their fateful mistake had been to remove all of the adult crabs while leaving the babies. Crabs are cannibals, and the adults generally control the population of their young by consuming them, so eliminating the adults allowed the small crabs to breed uncontrollably. All of the crabs captured in 2013 were tiny.

“This was the first time in a marine system that anyone had seen this kind of response,” Grosholz said. “It was a very large lesson.”

The green crab, known scientifically as Carcinus maenas, is a voracious predator of molluscs, worms and small crustaceans. The species, which can grow to be up to 4 inches wide, are native to the northeast Atlantic Ocean and the Baltic Sea, but made its way to Massachusetts in the hulls of ships in about 1817.

It has since spread to Australia, South Africa and South America through a variety of means, including ship’s hulls and seaplanes, and in the seaweed and other material that is shipped with live commercial fishing bait. Genetic testing has confirmed that the crabs found in Seadrift are related to the ones found in San Francisco Bay, which were shipped over from the East Coast in bait boxes 28 years ago.

Green crabs have extended their range along the West Coast to British Columbia, where they were discovered in 1999.

Although they have devastated shellfish operations and cost millions of dollars on the East Coast, they have not yet had a serious impact on the shellfish industry in Tomales Bay or elsewhere on the West Coast. Predation by native leopard sharks and rock crabs has forced the green crabs closer to shore, in wetland areas and lagoons, which is where scientists are mobilizing for battle.

Grosholz and his partners are considering introducing rock crabs and, possibly, leopard sharks, to prey on the crabs.

“It’s all about numbers with these guys,” he said. “If there are a lot of them, they eat everything else and they are a big problem for fisheries. We would like to have nature take care of itself.”

He started a citizen science program that will run through September to help marine ecologists capture and study the crabs until a solution is found.

For now, about 300 to 400 are being captured each day in 90 traps at six sites in the lagoon, reducing the population over the past four years to between 30,000 and 40,000. There is no market for green crabs — they contain very little meat — so the captured animals are frozen and given to a farmer in Bolinas for use as compost.

Grosholz said his work in Seadrift Lagoon is a lesson on the importance of checking ship hulls, cracking down on the dumping of ship ballast water and implementing tighter controls on sales of aquarium species.

“Unless you can, within a single year, get every last one of them, then you will see the same kind of explosion of green crab recruits that we saw,” he said. “The lesson going forward here is that we’re not going to be able to do much once an invasive species is established, so we should try even harder to keep them out of here in the first place.”